Laser processing can be conducted on numerous different workpieces using various lasers effecting a variety of processes. The specific types of laser processing of interest with regard to the present invention are laser processing of a single or multilayer workpiece to effect hole and/or blind via formation and laser processing of a semiconductor wafer to effect wafer dicing or drilling. The laser processing methods described herein could also be applied to any type of laser micromachining, including but not limited to removal of semiconductor links (fuses) and thermal annealing or trimming passive thick or thin film components.
Regarding laser processing of vias and/or holes in a multilayer workpiece, U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describe methods of operating an ultraviolet (UV) laser system to generate laser output pulses characterized by pulse parameters set to form in a multilayer device through-hole or blind vias in two or more layers of different material types. The laser system includes a nonexcimer laser that emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 100 ns, spot areas having diameters of less than 100 μm, and average intensities or irradiance of greater than 100 mW over the spot area. The preferred nonexcimer UV laser identified is a diode-pumped, solid-state (DPSS) laser.
U.S. Patent Application Publication No. US/2002/0185474 of Dunsky et al. describes a method of operating a pulsed CO2 laser system to generate laser output pulses that form blind vias in a dielectric layer of a multilayer device. The laser system emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 200 ns and spot areas having diameters of between 50 μm and 300 μm.
Laser removal of a target material, particularly when a UV DPSS laser is used, relies upon directing to the target material a laser output having a power, also referred to as fluence or energy density, that is greater than the removal threshold of the target material. Material removal can be effected by either a photo-chemical process, also called ablation, wherein the forces that hold atoms and molecules together are broken or by a photo-thermal process wherein the material is vaporized. A UV laser emits a laser output that can be focused to have a spot size of between about 5 μm and about 30 μm at the 1/e2 diameter. In certain instances, this spot size is smaller than the desired via diameter, such as when the desired via diameter is between about 50 μm and 300 μm. The diameter of the spot size can be enlarged to have the same diameter as the desired diameter of the via, but such enlargement would reduce the laser output energy density to the extent that it is less than the target material ablation threshold and cannot effect target material removal. Consequently, the 5 μm to 30 μm focused spot size is used and the focused laser output is typically moved in a spiral, concentric circular, or “trepan” pattern to form a via having the desired diameter. Spiraling, concentric circle, and trepanning processing are types of so-called non-punching via formation processes. For via diameters of about 70 μm or smaller, direct punching delivers a higher via formation throughput.
In contrast, the spot size of the output of a pulsed CO2 laser is typically larger than 50 μm and is capable of maintaining an energy density sufficient to effect formation of vias having diameters of 50 μm or larger on conventional target materials. Consequently, a punching process is typically employed when a CO2 laser is used to effect via formation. However, a via having a spot area diameter of less than 50 μm cannot be formed using a CO2 laser.
The high degree of reflectivity of copper at the CO2 wavelength makes very difficult the formation of a through-hole via using a CO2 laser in a copper sheet having a thickness greater than about 5 microns. Thus CO2 lasers can typically be used to form through-hole vias only in copper sheets that have thicknesses of between about 3 microns and about 5 microns or that have been surface treated to enhance the absorption of the CO2 laser energy.
The most common materials used in making multilayer structures for printed circuit board (PCB) and electronic packaging devices in which vias are formed typically include metals (e.g., copper) and dielectric materials (e.g., polymer polyimide, resin, or FR-4). Laser energy at UV wavelengths exhibits good coupling efficiency with metals and dielectric materials, so the UV laser can readily effect via formation on both copper sheets and dielectric materials. Also, UV laser processing of polymer materials is widely considered to be a combined photo-chemical and photo-thermal process, in which the UV laser output partly ablates the polymer material by disassociating its molecular bonds through a photon-excited chemical reaction, thereby producing superior process quality as compared to the photo-thermal process that occurs when the dielectric materials are exposed to longer laser wavelengths. For these reasons, solid-state UV lasers are preferred laser sources for processing these materials.
CO2 laser processing of dielectric and metal materials and UV laser processing of metals are primarily photo-thermal processes, in which the dielectric material or metal material absorbs the laser energy, causing the material to increase in temperature, soften or become molten, and eventually ablate, vaporize, or blow away. Ablation rate and via formation throughput are, for a given type of material, a function of laser energy density or fluence (laser energy (J) divided by spot size (cm2)), power density (laser energy density divided by pulse width (seconds)), pulse width, laser wavelength, and pulse repetition rate.
Thus, laser processing throughput, such as, for example, via formation on PCB or other electronic packaging devices or hole drilling on metals or other materials, is limited by the laser power available and pulse repetition rate, as well as the speed at which the beam positioner can move the laser output in a spiral, concentric circle, or trepan pattern and between via positions. An example of a UV DPSS laser is a Model LWE Q302 (355 nm) sold by Lightwave Electronics, Mountain View, Calif. This laser is used in a Model 5330 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc., Portland, Oreg., the assignee of the present patent application. The laser is capable of delivering 8 W of UV power at a pulse repetition rate of 30 kHz. The typical via formation throughput of this laser and system is about 600 vias each second on bare resin. An example of a pulsed CO2 laser is a Model Q3000 (9.3 μm) sold by Coherent-DEOS, Bloomfield, Conn. This laser is used in a Model 5385 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc. The laser is capable of delivering 18 W of laser power at a pulse repetition rate of 60 kHz. The typical via formation throughput of this laser and system is about 1000 vias each second on bare resin and 250–300 vias each second on FR-4.
Increased via formation throughput can be accomplished by increasing the pulse repetition rate at a pulse power that is sufficient to cause ablation as described above. However, for the UV DPSS laser and the pulsed CO2 laser, as pulse repetition rates increase, pulse power decreases in a non-linear fashion, i.e., twice the pulse repetition rate results in less than one-half the pulse power for each pulse. Thus for a given laser, there will be a maximum pulse repetition rate and hence maximum rate of via formation governed by the minimum pulse power needed to cause ablation.
Regarding dicing a semiconductor wafer, there are two common methods of effecting the dicing: mechanical sawing and laser dicing. Mechanical sawing typically entails using a diamond saw to dice wafers having a thickness of greater than about 150 microns to form streets having widths of greater than about 100 microns. Mechanically sawing wafers having a thickness that is less than about 100 microns results in cracking of the wafer.
Laser dicing typically entails dicing the semiconductor wafer using a pulsed IR, green, or UV laser. Laser dicing offers various advantages over mechanically sawing a semiconductor wafer, such as the ability to reduce the width of the street to about 50 microns when using a UV laser, the ability to dice a wafer along a curved trajectory, and the ability to effectively dice silicon wafers thinner than those that can be diced using mechanical sawing. For example, a silicon wafer having a thickness of about 75 microns may be diced with a DPSS UV laser operated at a power of about 8 W and a repetition rate of about 30 kHz at a dicing speed of 120 mm/sec to form a kerf having a width of about 35 microns. However, one disadvantage of laser dicing semiconductor wafers is the formation of debris and slag, both of which could adhere to the wafer and are difficult to remove. Another disadvantage of laser dicing semiconductor wafers is that the workpiece throughput rate is limited by the power capabilities of the laser.
The system and method described herein could also be used to monitor the parameters used to micromachine fusible links on a semiconductor wafer. A system designed to remove fusible links on a semiconductor wafer is described in U.S. Pat. No. 5,574,250 of Sun, et al, also assigned to the assignee of this patent application.
A goal of laser micromachining operations is to provide consistent quality of laser micromachined features over the entire workpiece. Some measures which define feature quality include the location, size, and shape of the feature. Further measures include sidewall angle, bottom texture, volume and texture of debris left in the feature after micromachining, and cracking near the edge of the feature, among others. One problem with laser micromachining operations as discussed herein is that, due to non-uniformities in the workpiece, performing the machining operations with the same laser parameters at two different locations on the workpiece can result in differences in feature qualities. Examples of workpiece differences that influence the results include differences in thickness, differences in workpiece flatness, and differences in surface preparation that makes the workpiece more or less reflective of laser power. These variations are not constant over the entire workpiece and can vary depending upon location down to the individual feature. Furthermore these variations can be repetitive from workpiece to workpiece in a given lot of workpieces due to normal variations in manufacturing tolerances.
Another phenomenon that affects the ability of the laser micromachining system to machine features is aging and/or damage to the optics used to direct the laser beam to the workpiece. As optical components age, they are subject to contamination, most notably from debris from the micromachining operation itself and damage from the high-power laser beams being transmitted through the optics. These and other forms of degradation can cause the laser spot projected onto the workpiece to change in size, shape, intensity or other characteristics, thereby changing the size, shape, depth, or other measures of the feature being micromachined in spite of the exact same parameters being used to control the laser beam.
Prior art systems for laser micromachining use real time controls that alter the parameters of the laser beam as the feature is being machined in an attempt to mitigate the effects of changes in the optics due to aging or damage. In some laser micromachining systems, in particular the systems referenced herein, a photodetector is used to monitor the laser power as the workpiece is being micromachined. The output from the photodetector is used to adjust the laser power in real time in an attempt to compensate for some of the sources of variability in laser power at the workpiece. This is typically accomplished by adjusting a variable attenuator in the optical path to change the laser energy transmitted by the optics to achieve a predetermined amount of energy for each machining operation. If the beam cannot be brought into agreement with a predetermined value, the operation is stopped and the operator is alerted that maintenance is required. The problem with this approach is that although it can help compensate for aging optics or other sources of variation in laser power, the fact that it is automatically compensating for possible deterioration of the laser or optics means that, unless the system is monitored, there is concealment of important information that could be used to trigger maintenance of the system before the system shuts down and valuable production time is lost.
Another issue with trying to maintain consistent quality of laser micromachined feature is that simply recording pre-selected laser parameters such as pulse repetition frequency (PRF) or pulse energy, among others, is not sufficient to characterize the laser parameters used in the micromachining process because the laser beam is typically not directed at the workpiece for the total nominal duration of the micromachining process. For instance, with the laser micromachining systems referenced herein, the laser beam is directed for a portion of the time it is pulsed onto a power meter which measures the power output of the laser beam. The system can then adjust a controllable attenuator in the optical path to cause the laser beam power to be raised or lowered to a pre-selected value. Typically this is used to compensate for reduce transmission by the optical components as they age. If the nominal laser energy value cannot be reached by adjusting the attenuator, the system generates an error signal. The problem is that unless the system is able to record the attenuator setting, the final laser power, and the number of pulses actually delivered to the workpiece, among other parameters, over the specific period of time the feature is being micromachined as opposed to being calibrated, there will exist no accurate record of the actual laser power directed at the workpiece during the micromachining of a particular feature.
Other sources of systematic, repeatable variations in laser micromachined features exist. For example, directing the laser beam to the appropriate locations on the workpiece to efficiently and effectively create multiple complex features can involve coordinating the movement of one or more subsystems, including motion control subsystems that move the workpiece and optical subsystems that move the laser beam. The combined result of these complex coordinated motions that describes the relationship between the workpiece and the laser beam is called the toolpath. The complexity of the toolpath is such that it is subject to repeatable transient behavior that affect the efficiency with which the laser beam interacts with the workpiece. Examples of the types of transient behavior that can affect the laser micromachining process include but are not limited to laser angle with respect to the workpiece and settling time. Another example of transient behavior is the change in laser pulse energy as a function of the pulsing duty cycle of the laser. Due to the toolpath layout and the dynamics beam motion, the time period between processing consecutive features can vary significantly. These delays affect the internal state of the laser, due to variations in stored energy in the lasing medium and thermal transients of optical cavity components. As a result of these laser transients, certain workpiece features may be processed with increased or decreased pulse energy, even if identical processing parameters (pulse repetition frequency, number of pulses, etc.) are applied to the feature. The result of these transient behaviors is systematic variations in feature quality despite identical laser parameter settings.
What is needed in laser micromachining features in a workpiece is, therefore, a method and system for monitoring, identifying, and optionally controlling the actual parameters used to micromachine a particular feature and storing parameter information to enable the system to retrieve the parameter information either in real time, as the feature is being machined or later, after the workpiece is complete.